This is a randomized, double-blind comparative safety and efficacy trial of sorivudine vs. acyclovir in the treatment of acute localized zoster and the effect on post-herpetic neuralgia in immunocompetent subjects. Patients will be assessed daily for up to 7 days, then on days 10, 14, 21, and 28. Thereafter, they will be contacted via telephone weekly until the end of the sixth month when they will return to the clinic for the last visit.